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Consider a cylindrical tank completely filled with water of height $1.6 \ m$ and cross-sectional area $0.5 \ m^2$. There is a hole in its side at a height of $90 \ cm$ from the bottom. Assume the cross-sectional area of the hole to be negligibly small compared to the cross-sectional area of the water tank. If a load of $50 \ kg$ is applied on the upper surface of water in the tank, then at the moment the hole is opened, the velocity of water coming out is:

 ($g = 10 \ m/s^2$)

The correct answer is

4 m/s

Calculating Efflux Velocity Under Load

This problem involves calculating the speed at which water exits a hole in a cylindrical tank. The calculation must account for both the height of the water above the hole and the additional pressure applied to the water's surface.

Given Parameters

  • Tank Height, $H = 1.6 \ m$
  • Cross-sectional Area of Tank, $A = 0.5 \ m^2$
  • Height of Hole from Bottom, $h_{hole} = 90 \ cm = 0.9 \ m$
  • Mass on Upper Surface, $M = 50 \ kg$
  • Acceleration due to Gravity, $g = 10 \ m/s^2$
  • Density of Water, $\rho \approx 1000 \ kg/m^3$

Determining Applied Pressure and Effective Height

An applied load creates additional pressure on the water surface, increasing the effective height driving the flow.

  1. Calculate the force exerted by the load:

    $ F = M \times g $

    $ F = 50 \ kg \times 10 \ m/s^2 = 500 \ N $

  2. Calculate the pressure applied on the water surface:

    $ P_{applied} = \frac{F}{A} $

    $ P_{applied} = \frac{500 \ N}{0.5 \ m^2} = 1000 \ Pa $

  3. Determine the equivalent height of water ($h_{applied}$) corresponding to this pressure:

    $ P_{applied} = \rho \times g \times h_{applied} $

    $ 1000 \ Pa = 1000 \ kg/m^3 \times 10 \ m/s^2 \times h_{applied} $

    $ h_{applied} = \frac{1000}{1000 \times 10} = 0.1 \ m $

  4. Calculate the actual height of water above the hole:

    $ h_{actual} = H - h_{hole} $

    $ h_{actual} = 1.6 \ m - 0.9 \ m = 0.7 \ m $

  5. Find the total effective height driving the flow:

    $ h_{eff} = h_{actual} + h_{applied} $

    $ h_{eff} = 0.7 \ m + 0.1 \ m = 0.8 \ m $

Calculating Efflux Velocity

Use the modified Torricelli's theorem, which relates efflux velocity to the effective height of the fluid.

The formula for efflux velocity ($v$) is:

$ v = \sqrt{2 \times g \times h_{eff}} $

Substitute the values:

$ v = \sqrt{2 \times 10 \ m/s^2 \times 0.8 \ m} $

$ v = \sqrt{16} \ m/s $

$ v = 4 \ m/s $

The velocity of water coming out of the hole is 4 m/s.

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